If (1) is true, it would prevent them from spreading all over the galaxy, so they can't find us (to destroy us) this early in our evolution, a hundred years after we used radio. They might still destroy us relatively soon,, but they wouldn't be present in every star system. (Also, the fact we evolved in the first place might be evidence against this scenario.)
If (2) is true, they would have to somehow destroy all other life in the galaxy without risking that destroying-mechanism value-drifting or being used by a rival faction. This might be hard. Also, their values might not endorse destroying aliens without even contacting them.
If (1) is true the aliens should fear any type of life developing on other planets because that life would greatly increase the complexity of the galaxy. My guess is that life on earth has, for a very long time, done things to our atmosphere that would allow an advanced civilization to be aware that our planet harbors life.
For a moment lets assume there is some alien intelligent life on our galaxy which is older than us and that it have succeeded in creating super-intelligent self-modifying AI.
Then what set of values and/or goals it is plausible for it to have, given our current observations (I.e. that there is no evidence of it`s existence)?
Some examples:
It values non-interference with nature (some kind of hippie AI)
It values camouflage/stealth for it own defense/security purposes.
It just cares about exterminating their creators and nothing else.
Other thoughts?